The present invention relates to improvements in set screws for use with apparatus for correcting orthopedic deformities and, in particular, for use in spinal osteosynthesis.
Surgically implanted spinal osteosynthesis apparatus often include rods which are secured along at least a portion of the spine by a system of hooks, bone screws, including sacral screws and pedicle screws and transverse connectors for the purpose of stabilizing and adjusting spinal alignment. In a very basic apparatus of this type, the hooks and bone screws include a spinal rod bore extending through a ring or body or head of the hook or screw. The screws are screwed into the pedicle portion of the vertebra at desired locations and a spinal rod is then extended through the spinal rod bore in each bone screw.
Where the bone screw has a rod receiving ring and the rod is to be fixed in position in the ring, a set screw is inserted in a threaded bore extending through a wall of the ring, so as to engage the rod, and is then tightened to fix the translational and rotational relationship of the rod within the ring. The rods may then be bent or shaped to maintain an adjacent portion of the spine in a desired configuration, to provide support to the spine and to exert desired corrective or stabilizing forces on the spine.
A slightly more complicated system uses transverse connectors in association with the bone screws to secure the spinal rods. The transverse connectors include an arm and a head. The head has a spinal rod bore extending therethrough. The arm of the connector is inserted through the spinal rod bore in the pedicle screw then the spinal rod may be inserted through the spinal rod bore in the transverse connectors. A threaded bore extends through the head of the connector perpendicular to the axis of the spinal rod bore. Once the rod is inserted through the bore in the transverse connectors the set screws are inserted through the threaded bores and tightened to fix the relative position of the rod within the spinal rod bore, and set screws are inserted in the threaded bores and tightened to fix the position of the transverse connector with respect to the pedicle screws.
The pedicle screws and transverse connectors may be of the closed type as discussed above or of an open end type wherein the head of the screw or connector generally incorporates a U-shaped groove. One type of open end bone screw is shown in the U.S. Patent of Cotrel No. 5,005,562. The device in the Cotrel patent has threaded interior surfaces on the two upright branches that form the rod receiving channel therebetween and which receive a threaded set screw having a rod engaging point and outer ring. The set screw in Cotrel is tightened against the rod by advancing the set screw along the threads. However, this system has limitations. In particular, the ability of the set screw of Cotrel to grip and hold the rod is heavily dependent on the torque applied to the set screw during installation. Unfortunately, the torque is limited because too much torque will cause the branches to spread, thereby allowing the set screw to loosen and the implant to fail. Such failure can also occur when forces are applied to the implant during use, such as at time of muscular stress or during accidents when the back is jolted. To try to overcome this problem associated with the Cotrel device, the implant branches and set screw are increased in size to add strength and/or a retention ring is placed around the outside of the branches to reduce the likelihood of expansion. However, the strengthening adds substantial bulk to an implant and a ring adds bulk and complexity to the implant. In implants, it is important to try to reduce bulk rather than add to it, as it is desirable for the implants to be as low profile as possible.
Rather than have a pair of branches joined only by a set screw or by a set screw and an exterior ring, a cap has been proposed which mates with the branches on opposite sides of the cap to prevent the branches from expanding radially outward upon application of torque to the set screw. The cap also closes off the open end of the bone screw after the rod is placed in the groove in the bone screw. The set screw is then inserted in a threaded bore in the cap and tightened to fix the position of the transverse connector with respect to a respective bone screw. A substantial torque can then be applied to the set screw while held in the surrounding threads of the cap without expanding the bone screw branches.
Various implants such as hooks, pedicle screws and transverse connectors with which the set screw of the present invention is used may be of the closed type, as discussed above, or of an open end type, such as described above, wherein the head of the hook screw or connector generally incorporates a U-shaped groove or slot, an upper end of which may be closed off by a cap after a rod is placed in the open end so as to complete the rod bore. A threaded screw bore for the set screw typically extends through the cap.
The efficacy of the set screw is critical to the overall performance and efficiency of the osteosynthesis apparatus. The set screw must firmly secure the spinal rod or the arm of transverse connectors to prevent rotational or translational movement of the rod or arm after installation. Due to the nature of use of the set screw, it is important that the set screw be relatively small yet constructed to receive sufficiently high torque to firmly set the set screw and hold the rod. The set screw must also be easily manipulated to permit relatively rapid insertion and tightening during surgical procedures. It is also preferable that after insertion, no portion of the set screw extends beyond the threaded bore into which the set screw is inserted. The remaining portion of the set screw should be removable to facilitate disassembling of the osteosynthesis apparatus at any time. It is desirable that the set screw take advantage of physical penetration into the rod so as to improve the strength of the connection to resist axial movement of the rod relative to the set screw over that provided only by abutting friction.
Set screws have been previously developed with breakoff heads or stems which break off after the set screw is inserted through a threaded bore and tightened to a preselected torque. Preferably, no portion of the set screw that remains after the head or stem breaks off extends above or beyond an outer edge of the threaded bore. However, prior art set screws normally have undesirable burrs that are left after the head breaks off that must be removed, thereby making the procedure more difficult or alternatively such burrs may lead to irritation of the patient, if not removed. Often, after installation, a set screw must be removed to reposition a rod or fix a broken apparatus. Prior art set screws have been difficult to remove after the head or stem is broken off. Consequently, it is desirable to have a set screw that can be comparatively easily removed even without a head.
It is also desirable to have a set screw that has an axially aligned tip that penetrates relatively deeply into a rod for preventing movement along or around the rod of an associated implant once tightened, but also includes structure that helps prevent rocking or translational movement of the set screw relative to the point of penetration. Rocking or movement of the screw relative to the location of penetration weakens the grip provided by the tip in the rod and the prevention of such movement substantially strengthens the juncture of the screw and the rod. The set screw tip, such as a point can only penetrate deeply into the rod if sufficient torque can be applied to the set screw to do so. In general greater torque is available due to greater bulk or due to special construction that allows greater strength without adding bulk. The latter is preferred in implants.
It is also desirable to have a set screw that is relatively easy to manufacture to help keep the cost of the screws down.
In general, there is still a need for an improved set screw which is quite strong in size, reliable in securing an osteosynthesis apparatus in place without burrs or high profile, is easily removable and is relatively small yet easily manipulable to facilitate its insertion and removal.